Effects of forebody shape on directional static stability of a tailless chined forebody-wing configuration

The tailless chined forebody-wing configuration improves stealth performance by eliminating the vertical tail. However, as the angle of attack (AoA) increases, its directional static stability transitions from instability to stability and reverts to instability, challenging flight control. Therefore, this study employed numerical simulations and wind tunnel experiments to elucidate the flow mechanisms and explore geometric improvement strategies. At low AoAs, the directional instability arises from the attached flow over the chined-body. At moderate AoAs, asymmetric vortices form over the forebody and the windward vortex overpowers the leeward one, creating asymmetric suction on the forebody and leading to directional stability. At high AoAs, the premature breakdown of the windward vortex causes a reversion to instability. Furthermore, parametric studies of forebody geometry reveal that increasing forebody upper surface height ratio (Ru) modifies local contraction/expansion rates, redistributes pressure gradients, and alters spatial constraints for vortex roll-up, thereby weakening the windward vortex while strengthening the leeward one to reduce asymmetry. Coupled with the variation in forebody upper surface area, an intermediate Ru yields the widest AoA range of stability. For the chine-edge modifications, at moderate AoAs, sharp edges enforce a geometrically fixed separation and promote the premature development of the windward vortex, thereby enhancing flow asymmetry and advancing stability onset. Conversely, at high AoAs, blunt lower edges delay the reversion to instability by mitigating the geometric discontinuity, which promotes a more gradual shear-layer roll-up, and mitigating the adverse pressure gradient, thereby weakening the leeward vortex and delaying windward vortex breakdown.